I do not know why you cannot install the plugins. Does Premiere Pro installed on the system disk?Unfortunately, I cannot test with Premiere Pro CC2015 and Windows 7 as I do not have a similar system.What you can do is to unzip that file to C:\Program Files\Adobe\AdobePremiereProCC2015\Plug-Ins\Common\GoPro\GoProVRHorizon.prm + GoProVRLayers.prm + GoProVRReframe.prm

So, it is miracle or what, but I updated to 2018 PP and AE, reinstalled the VR plugin and it works! With some issues, but works!When I create 1080p sequence and apply VR Reframing there is only FOV working correct, other options do not work.When I create 720p sequence all options work fine. But it's 720p...I badly need to upgrade the whole PC